introduce a lalpulsar.MSUN_TEMPO2 constant
@matthew-pitkin as discussed by e-mail, it seems like a good idea having a separate constant defined to ensure full numerical compatibility of the ReadPulsarParFile code with TEMPO2 and decouple it from lal's own solar mass.
cc @karl-wette @jolien-creighton @michal-bejger @yousuke.itoh